The prior art discloses determining the Hg vapor pressure of a Hg low-pressure discharge lamp, in order that this can be taken into consideration when controlling the Hg low-pressure discharge lamp. In this case, the Hg vapor pressure is calculated indirectly from the temperature by means of attaching a temperature sensor to the lamp bulb or to the light. The temperature sensor is preferably arranged in the vicinity of or directly at the so-called cold spot. In the case of amalgam lamps, the temperature sensor is preferably attached in the vicinity of the amalgam base.
For the purpose of control, the temperature sensor is connected to a control unit, e.g. a so-called DALI unit, which forwards the parameters required for the lamp operation to an electronic ballast resistor. The control unit may also be integrated directly into the electronic ballast resistor.
However, the use of a temperature sensor has the following disadvantages:
If the cold spot is situated at a distinct position, a temperature sensor may be attached at this position using a suitable heat-conductive paste, for example. Although the temperature at this position may be determined thus, and therefore the prevailing Hg vapor pressure may be deduced indirectly after suitable calibration, such a measuring system nonetheless involves an undesired time lag due to heat conductivity and heat capacities of the temperature sensor and the discharge tube. Determination of the Hg vapor pressure is therefore delayed.
Secondly, the exact location of the cold spot may vary depending on the operating conditions of an Hg low-pressure discharge lamp: particularly critical are applications that are exposed to temporary drafts, applications at very low outside temperatures, e.g.<−20° C., or applications in which the lamps are operated dynamically, in particular where states of low dimming, for example>90% nominal power consumption, alternate with states of high dimming, for example<10% nominal power consumption. Depending on the initial state and the time duration of the dimming sequence, the location of the cold spot may move. Merely by way of example, reference is made to so-called T5 lamps (featuring cold-spot technology), in which the cold spot moves from the original location at the edge of the socket to the center of the lamp when the discharge tube cools. Without knowing the location of the cold spot, the Hg vapor pressure cannot be precisely determined, and it is therefore impossible to specify reliable or correct lamp operating parameters. Consequently, reliable operation of the lamp cannot be guaranteed.